A sequesterer is a compound which is able to interact noncovalently with other molecules to form stable inclusion complexes. A well known class of sequesterers are the three cyclodextrins: .alpha.-, .beta.-, and .gamma.-cyclodextrin which contain, respectively, six, seven, and eight anhydroglucose (C.sub.6 H.sub.10 O.sub.5) units. These molecules are doughnut-shaped rings having a hollow cavity of a specific volume. The polar hydroxyl groups are oriented to the outside of the rings, giving the outer surface a hydrophilic nature. In contrast, the internal cavity has a hydrophobic (lipophilic) nature. Because of this unique structure, cyclodextrins, as the "host" molecules, are able to hold "guest" molecules of suitable size (generally of a molecular weight between 80 and 250), shape, and hydrophobicity within their cavity. Although cyclodextrins are very expensive to produce and are of limited availability, a number of important sequestering uses have been discovered for them. See, e.g., D. E. Pszczola, "Production and Potential Food Applications of Cyclodextrins," Food Technology, January 1988, pp. 96-100. It would be very desirable to provide sequesterers that are easily produced and that accommodate guest molecules of widely-varying size, shape, and hydrophobicity.
Cyclodextrins are derivatives of starch, a plant material formed of anhydroglucose polymers. Starch is a member of the class of compounds known as polysaccharides in that it contains three or more saccharide units. It is also a member of the broader class of compounds known as carbohydrates. Starch occurs naturally in plants in the form of granules having an average size of about 5 to 100 microns and containing thousands of individual starch molecules bound tightly together. Starch molecules exist as lightly-branched chains consisting predominantly of .alpha.-1,4-linkages between the anhydroglucose units, known as amylose, and as highly-branched chains consisting of both .alpha.-1,4- and .alpha.-1,6-linkages, known as amylopectin. Differences between amylose and amylopectin have been studied by first rupturing the starch granule and then separating the two fractions. This separation is often accomplished by precipitating amylose as a complex with butanol. Amylose has also been reported to form complexes with aliphatic alcohols such as isopropyl and isoamyl, as well as a variety of other organic compounds. R. Whistler et al. (eds.), Starch: Chemistry and Technology, 2d ed., Academic Press, Inc., 1984, pp. 154-155 and 260.
Based on these, and other, studies, it is believed that amylopectin grows in clusters having a length of about 50 to 70 Angstroms. Additional studies have led to the discovery that the amylopectin molecule contains three distinct types of chains which have been designated as A-chains, B-chains, and C-chains. See D. Manners, "Recent Developments in Our Understanding of Amylopectin Structure," Carbohydrate Polymers, Vol. 11, 1989, pp. 87-112. The single C-chain contains the only reducing group of the molecule and is the chain from which all other chains branch. B-chains are linked to two or more other chains. A-chains are bound to only one other chain. It is believed that A-chains contain about 12 to 16 anhydroglucose units and are generally confined to individual clusters. B-chains show modality in their size distribution. Three modes, which are designated B.sub.1, B.sub.2, and B.sub.3, comprise most of the B-chains and average about 22, 45, and 70 anhydroglucose units, respectively. It is believed that the B.sub.1 fraction is confined to a single cluster, but that the other B-fractions extend into other clusters. A fourth mode of B-chains, which are as long as 200 anhydroglucose units and which extend beyond three clusters, have been found and are designated B.sub.4.
The exact structure of starch on a molecular level is still under investigation. It is believed that at least some unmodified starch exists on a molecular level in the form of a double helix-two, long, winding chains in close association with each other. Various theories regarding the nature of this association have been postulated: some scientists believe the chains are intertwined in a parallel arrangement while others believe that the chains are intertwined in an antiparallel arrangement. It is also believed that, at least in some types of starch, a portion of the amylose exists in the form of a single helix inclusion complex-a single winding chain containing six anhydroglucose units per turn wrapped around a lipid molecule. See T. Galliard, Starch: Properties and Potential, John Wiley & Sons, 1987, pp. 69-75.
One method frequently used to investigate the structure of the starch molecule is to observe its response to exposure to electromagnetic radiation. When unmodified starch is exposed to polarized light, it exhibits a birefringence (judged empirically by the presence of a characteristic "Maltese Cross" pattern) indicating an orderly, crystalline-like structure. It is believed that the amylopectin fraction of starch is primarily responsible for this crystallinity.
Starch also exhibits characteristic diffraction patterns when exposed to x-ray radiation. Cereal starches exhibit a pattern identified as the A-pattern, tuber and root starches exhibit the so-called B-pattern, and starches of the Liguminosae family exhibit the C-pattern (there is no relationship between the use of the letters A, B, and C in connection with types of chains and the use of the same letters in connection with types of diffraction patterns, i.e., an A-chain does not imply an A-diffraction pattern). It is believed that the single helix amylose inclusion complexes present in some unmodified starches generate a V-pattern, but it is not discernible because of the limited presence of these complexes. Id. at 74.
Returning to the macro level, unmodified starch granules are insoluble in cold water, but can be dissolved by heating in water at a temperature of about 70.degree. to 90.degree. C. at atmospheric pressure. At this point the granules gradually swell and rupture and the individual molecules pass into solution. This process by which starch granules swell and rupture is alternatively referred to as gelatinization, pasting, or cooking. As the starch gelatinizes, its crystallinity disappears and its birefringence is lost. It is believed that, during gelatinization, those starch molecules existing in the form of a double helix disengage to form two single-stranded polymer chains.
The major commercial source of starch is corn (also known as maize), but potatoes, wheat, barley, rice, and tapioca are also important sources. The relative amounts of amylopectin (branched chains) and amylose (primarily unbranched chains), as well as the average number of anhydroglucose units in a molecule (commonly known as the Degree of Polymerization, or D.P.) varies with the species of plant. For example, common dent corn starch contains about 72% amylopectin and 28% amylose; waxy maize corn starch contains nearly 100% amylopectin; and a common high amylose corn starch contains about 45% amylopectin and 55% amylose (all percentages are given on a weight percent basis unless otherwise indicated). Corn starch amylopectin has a D.P. of about 300,000 to 3,000,000 while amylose has a D.P. of about 800 to 8,000.
Corn starch is often modified chemically to alter its physical properties for a given application. One common modification is to substitute other chemical groups onto the hydroxyl groups of the starch molecules. The amount of substitution is expressed as the Degree of Substitution, or D.S. A starch molecule having one substituted group per anhydroglucose unit is defined as having a D.S. of one. A molecule having one substituent per 2 anhydroglucose units has a D.S. of 0.5., and so on. Another common modification is to treat the starch with an agent such as acid or enzyme to cleave some of the bonds between the anhydroglucose units to produce shorter chain segments and thereby reduce the average D.P. of the starch molecules. Starches having a reduced D.P. are said to be hydrolyzed or convertecL They are often described in terms of their Dextrose Equivalent, or D.E., which is defined for an individual molecule as 100.div.D.P. Accordingly, the monosaccharide dextrose, also known as glucose, has a D.P. of 1 and a D.E. of 100. Dextrose Equivalent is a convenient measure of the sweetness of a starch derivative and is widely used in the corn refining industry. In a mixture of molecules having different D.P.'s, D.E. can be viewed as a non-weighted average of the D.E.'s of the individual molecules. Depending upon the population distribution, two mixtures having the same average D.P. can have different D.E.'s, as shown by the following example.
Assume mixture A contains 6 starch derivative molecules, each one having a D.P. of 2 and a D.E. of 50. The average D.P. of the mixture is, of course, 2: [(2+2+2+2+2+2).div.6] and the D.E. of the mixture is 50: [(50+50+50+50+50+50 ).div.6]. Now consider mixture B which also contains 6 molecules, but of the following distribution: The first molecule has a D.P. of 4 and a D.E. of 25; The second molecule has a D.P. of 3 and a D.E. of 33.33; The third molecule has a D.P. of 2 and a D.E. of 50; And the remaining three molecules each has a D.P. of 1 and a D.E. of 100. This mixture has an average D.P. of 2: [(4+3+2+1+1+1).div.6], the same as mixture A. But mixture B has a D.E. of 68: [(25+33.33+50+100+100+100).div.6], and would be sweeter to the taste than mixture A.
Although the cyclodextrins are the only starch derivatives currently used commercially as sequesterers, a large number of starch derivatives have been disclosed for other uses. For example, Eastman et al., U.S. Pat. No. 4,465,702, issued Aug. 14, 1984, disclose a cold-water-soluble, granular starch derivative prepared by heating ungelatinized starch in a slurry of selected aqueous alcohols under high pressure. Rajagopalan et al., U.S. Pat. No. 5,037,929, issued Aug. 6, 1991, disclose a granular cold-water-soluble starch prepared by heating a slurry of granular starch, water, and a polyhydric alcohol under atmospheric pressure. Both the Eastman et al. and Rajagopalan et al. starch derivatives exhibit a V-type x-ray diffraction pattern. See J. Jane et al., "A Granular Cold Water-Soluble Starch Gives A V-Type X-Ray Diffraction Pattern," Carbohydrate Research, Vol. 150, 1986, pp. C5-C6. Neither the Eastman et al. process nor the Rajagopalan et al. process significantly affects the D.P. of the starch molecules. These starch derivatives are alleged to be particularly useful in food systems of the type which set or gel upon standing. Sugimoto, U.S. Pat. No. 3,799,805, issued Mar. 26, 1974, discloses a process for producing granular dextrins. The dextrins are prepared by heating granular starch with acid in an aqueous solution of an organic solvent such as propanol, ethanol, methanol, acetone, and fatty acids. Sugimoto teaches that the resulting dextrins are useful in the production of maltodextrins.